I've just replaced my ECU and notice that the Idle speed kinda moves. I left the car running and then notice that the idle went from 500 to about 900 and then back to 500. It happens every 3-5 mins. I didn't really take notice to this before the ECU incident. So was mine damge in anyway or reseted improperly?

Any insight, help or suggestion would be great.
thanks,
Mike